1. Field of the Invention
This invention relates to Magnetic Resonance (MR) imaging, and more specifically, pole faces of open magnets used in MR imaging.
2. Description of Related Art
Typically in magnetic resonance (MR) imaging, a patient is inserted into a `closed` cylindrical shaped magnet. This arrangement does not allow the Physician access to the patient to perform procedures interactively during imaging.
`Open` magnets have been employed to allow access to the patient during imaging. These however, suffer from the problems of creating a strong, uniform, homogeneous magnetic field over an imaging volume, especially when time-changing magnetic field gradients are applied.
When ferromagnetic materials are used, the surface of the open magnet emanating the magnetic field is called a pole face. It receives magnetic flux from a magnetic source attached to it. The magnetic sources may, or may not be connected with a low reluctance (high magnetic permeability) ferromagnetic flux closure, completing a "C" shaped flux path.
Problems arise when a rapidly time-changing magnetic field is applied to the pole faces. Since the pole faces are constructed of electrically conducting materials, eddy currents are created, which in turn, create magnetic fields that oppose the applied magnetic field.
Application of the time-changing magnetic field also causes a secondary, residual, magnetic field to be created in the pole face material, which remains after an applied magnetic field is removed. This is described by the hysteresis of the material.
Both eddy currents, and the residual magnetization distort the magnetic field, and therefore, the image created from that magnetic field.
U.S. Pat. No. 5,283,544 Sakurai et al., Feb. 1, 1994 explains a device aimed at reducing eddy currents in an open MR imaging device by minimizing the path of the eddy current. This device employs pole faces constructed from blocks. The eddy currents would not flow around the entire pole face, but just in each block individually.
Sakurai also constructed each block of laminate sheets aligned parallel to the induced time-changing magnetic field. This allowed greater permeability of the magnetic flux in a direction parallel to the laminate sheets.
Sakurai's construction did reduce eddy currents and residual magnetization, however, these effects may be further reduced, and further improve imaging.
Currently, there is a need to further reduce induced eddy current and residual magnetization in open MR imaging machines to reduce artifacts in images created.